It has been conventionally known a class-D power amplifier which performs power amplification by a class-D operating method which converts an inputted analog signal into a pulse-width modulation (PWM) signal and on-off controls a switching element of an output stage by the pulse-width modulation signal.
FIG. 4 and FIG. 5 illustrate structures of class-D amplifiers as examples of the conventional class-D power amplifier. Illustrated in FIG. 4 is an example of a half-bridge connection, and illustrated in FIG. 5 is an example of a full-bridge connection.
First, in the case of the half-bridge connection, as illustrated in FIG. 4, one speaker SPx is connected to one class-D power amplifying unit Dx.
Then, in the class-D power amplifying unit Dx, an analog audio signal to be amplified is inputted as an input signal IN, voltage thereof is PWM-modulated by the PWM modulator X, and is converted to a PWM signal. Then, this PWM signal is supplied to each of a switching element Sxa provided between a positive side power supply (+V) and an output signal line and a switching element Sxb provided between a negative side power supply (−V) and the output signal line, so as to complementarily drive the switching elements Sxa and Sxb. That is, the switching elements Sxa and Sxb are driven so that, for example, when the PWM signal is at a high level, the switching element Sxa turns on and meanwhile the switching element Sxb turns off, and conversely when the PWM signal is at a low level, the switching element Sxa turns off and meanwhile the switching element Sxb turns on.
In this manner, electric current flows from the positive side power supply to the output signal line at a timing the switching element Sxa conducts, and electric current flows from the output signal line to the negative side power supply at a timing the switching element Sxb conducts. By passing this current varying according to high/low of the PWM signal through a low-pass filter formed by a coil Lx and a capacitor Cx, a carrier signal used for the PWM conversion can be removed, thereby demodulating it to an analog audio signal which is a power-amplified input signal IN (the maximum voltage which can be outputted depends on the voltage of the power supply). Therefore, by supplying the signal passed through the low-pass filter to the speaker SPx via an audio output terminal, a speaker SPx can be driven with the analog audio signal which is the amplified input signal IN. Note that the signal returning from the speaker to the class-D power amplifying unit Dx flows to the ground of the amplifier.
Further, in the case of the full-bridge connection, as illustrated in FIG. 5, one speaker SPx is connected to the two class-D power amplifying units Dx, Dy.
Then, in the class-D power amplifying unit Dx, similarly to the case of FIG. 4, an analog audio signal which is a power-amplified input signal IN is supplied to the speaker SPx.
On the other hand, to the class-D power amplifying unit Dy, the input signal IN is inverted between positive and negative by an inverter E and inputted. Then, this inverted signal is PWM-modulated in a PWM modulator Y similarly to the case of the class-D power amplifying unit Dx, and drives the switching elements Sya and Syb complementarily with a PWM signal as a modulation result thereof. Thus, by passing through the low-pass filter formed by a coil Ly and a capacitor Cy, an analog audio signal which is exactly the same as that on the class-D power amplifying unit Dx side except that it is inverted between positive and negative can be supplied to the speaker SPx. Note that the power supply is common between the class-D power amplifying unit Dx and the class-D power amplifying unit Dy.
Here, by connecting two terminals of the speaker SPx to the class-D power amplifying units Dx and Dy, respectively, the speaker SPx can be driven with a signal of difference of the analog audio signals supplied from the class-D power amplifying units Dx and Dy, respectively. The analog audio signals supplied respectively from the class-D power amplifying units Dx and Dy are signals inverted between positive and negative as described above, and thus by taking the difference, the speaker SPx can be driven by voltage that is two times the case of the half-bridge connection.
Such a class-D power amplifier is known as an amplifier with quite high efficiency. Further, as technology related to such a class-D power amplifier, for example, ones described in PTL1 and PTL2 are known.
Further, as the speaker connected to the class-D power amplifier, for example, a low-impedance single speaker whose impedance is 4Ω or 8Ω is widely used. However, besides this, it is also practiced to connect plural speakers in parallel by a system called constant voltage system.
Such a constant voltage system transmits an audio signal with high voltage and low current from the amplifier to respective speakers, converts them to a low voltage signal by a step-down transformer provided in an input unit of each speaker, and drives the speaker with this low voltage signal. That is, a high-impedance signal line is used as a main signal line connecting the amplifier and the respective speakers, and the plural speakers are connected in parallel thereto, respectively, via the step-down transformers.
Such a constant voltage system is described in, for example, NPL1.